A portable power tool such as an impact driver includes a housing including a body part having a substantially cylindrical shape and extending back and forth and a handle part connected to the body part so as to form a substantial T shape, when seen from the side. The body part accommodates therein a driving source such as a motor and the like, a power transmission mechanism part that transmits a driving force of the driving source and a tip tool holding part to which the driving force of the driving source is transmitted from the power transmission mechanism part. The handle part includes a switch for controlling the driving source. Such an impact driver is disclosed in JP-A-2008-62347 and JP-A-2010-99823. Here, the related-art impact driver, which is an example of the power tool, will be described on the basis of FIGS. 13 to 15. Incidentally, FIG. 13 is a left side view of the related-art impact driver, and FIG. 14 is a right side view of the related-art impact driver.
In FIGS. 13 and 14, an impact driver 101 has a housing 102, which is an outer frame forming an outward shape, and a hammer case 103 that is mounted at a front of the housing. The housing 102 is configured by a substantially cylindrical body part 102a extending back and forth, a handle part 102b connected to the body part 102a such that it forms a substantial T shape, when seen from the side, and a battery attaching part 102c that is formed to expand at the lower of the handle part 102b. A trigger 106 for starting a motor and adjusting a rotating speed of the motor and a changeover lever 108 for switching a rotating direction of the motor are provided above the handle part 102b of the housing 102 and just below the body part 102a. A battery 140 such as a nickel cadmium battery and a lithium ion battery is detachably mounted to a lower side of the housing 102. A side of the body part 102a of the housing 102 is provided with an air introduction port 111 for introducing air for motor cooling and air discharge ports 112a, 112b for discharging the inside air.
FIG. 15 is a longitudinal sectional view showing an internal structure of the impact driver 101 of the related art. In the body part 102a of the housing 102, a motor 105 that is the driving source, a planetary gear mechanism 120 configured to decelerate a rotation output of the motor 105 and a striking mechanism part 121 configured to convert a rotating force of the motor 105 decelerated by the planetary gear mechanism 120 into a rotating striking force and to transmit the rotating striking force to an output shaft 130 for holding a tip tool are accommodated. The handle part 102b of the housing 102 is provided with the switch 107 and the trigger 106 for operating the switch 107 is provided such that it is exposed to the outside beyond the housing 102. The battery 140 is mounted to the battery attaching part 102c of the housing 102. The changeover lever 108 can be transversely moved and selectively protrudes from a left or right surface of the body part 102a. When an operator pushes the protruding part of the changeover lever 108 and protrudes the lever from the other end-side, the rotating direction of the motor 105 can be switched between a positive direction (e.g., an engaging direction) and a reverse direction (e.g., a disengaging direction).
With the impact driver 101, when the operator turns on the trigger 106 to thereby start the motor 105, the rotating force of the output shaft 112 of the motor 105 is decelerated by the planetary gear mechanism (deceleration mechanism) 120 and is then transmitted to a spindle 127, so that the spindle 127 is rotated at predetermined speed. Here, the spindle 127 and a hammer 124 are connected to each other by a cam mechanism. The cam mechanism includes a V-shaped spindle cam groove 125 formed on an outer periphery of the spindle 127, a V-shaped hammer cam groove 128 formed on an inner periphery of the hammer 124 and a ball 126 that is engaged with the cam grooves 125, 128. The hammer 124 is always urged in a tip direction (a forward direction) by a spring 123, and in a non-operation state, the hammer 124 is spaced apart at an interval from an end face of an anvil 129 by engagement of the ball 126 and the cam grooves 125, 128. Convex portions are symmetrically formed at two positions on rotary surfaces on which the hammer 124 and the anvil 129 are opposed to each other, respectively. Incidentally, the anvil 129 is integrally formed with the output shaft 130 configured to detachably hold a bit (not shown), which is the tip tool, and a tip of the output shaft 130 is formed with a bit mounting hole (a tip tool holding part) 130a having an axially perpendicular hexagonal section.
Incidentally, when the spindle 127 is rotated at the predetermined speed, as described above, the rotation of the spindle 127 is transmitted to the hammer 124 through the cam mechanism and the convex portion of the hammer 124 is engaged with the convex portion of the anvil 129 to thus rotate the anvil 124 while the hammer 124 does not make a half turn. However, at this time, when a relative rotation is caused between the hammer 124 and the spring 123 by an engaging reactive force, the hammer 124 starts to retreat towards the motor 105 while compressing the spring 123 along the spindle cam groove 125 of the cam mechanism.
When the hammer 124 retreats and the convex portion of the hammer 124 goes over the convex portion of the anvil 129 and the engaged state therebetween is thus released, the hammer 124 is moved forwards by the urging force of the spring 123 while being rapidly accelerated in the rotating direction and in the forward direction by virtue of the rotating force of the spindle 127 and action of the elastic energy accumulated to the spring 123 and the cam mechanism, and the convex portion of the hammer 124 is again engaged with the convex portion of the anvil 129, so that they are integrally rotated. At this time, since the strong rotating striking force is applied to the anvil 129, the rotating striking force is transmitted to a screw (not shown) through the bit mounted to the anvil 129. After that, the same operation is repeated, so that the rotating striking force is intermittently transmitted to the screw from the bit and the screw is thus screwed into a material to be engaged (not shown) such as wood and the like.
Incidentally, according to the impact driver 101, the operator performs the operation while turning on/off the trigger 106 by using an index finger with gripping the handle part 102b of the housing 102. In order to prevent the operator's finger gripping the handle part 102b from sliding or to make better a gripping feeling to thereby improve the operability and workability, a surface of the housing 102 is formed with a soft layer consisting of elastomer and the like softer than a hard layer configuring a main body part of the housing 102. Also, a tip of the body part 102a of the housing 102 is attached with a protector 104 consisting of soft elastomer and the like, which prevents a damage of a member to be engaged such as a building material and also prevents the impact driver 101 from sliding when it is put on an inclined surface. Also, a part of the handle part (grip part) 102b, which is gripped by the operator, is provided with a sliding prevention mechanism such as convex surface such that the operator can always comfortably grip the same.